An electronic paper is known as an electronic medium replacing a paper, and recently, an electronic paper has been developed actively.
Because an electronic paper is characterized in that a display device is used like a paper, a characteristic is required which is different from that of a conventional apparatus such as a CRT (Cathode Ray Tube) or a liquid crystal display. For example, a characteristic is required such that it is a reflection-type display device, has a high white reflectance or high contrast ratio, is capable of providing a highly fine display, providing a display with a memory effect, is capable of operating at a low voltage, is thin and light, and is inexpensive. Among these, in particular, there is a high need for a white reflectance or contrast ratio comparable to that of a paper, and further a color display, as a characteristic associated with a display quality.
Until now, for example, a reflection-type liquid crystal display device using a reflection-type liquid crystal, an electrophoretic display device using electrophoresis, a device using a toner migration, and the like are proposed and known as display devices for an electronic paper application. However, it may be very difficult for any of the above-mentioned devices to conduct multicolor display while a white reflectance and a contrast ratio are retained. Generally, a color filter is provided to conduct multicolor display, but when a color filter is provided, such a color filter absorbs light to degrade its reflectance. In addition, a color filter divides one picture element into three parts, for example, red (R), green (G), and blue (B), whereby a white reflectance of a display device is degraded and accordingly, a contrast ratio thereof is degraded. When a white reflectance and a contrast ratio are considerably degraded, a visibility may be deteriorated significantly and its use as an electronic paper may be difficult.
Japanese Patent Application Publication No. 2003-161964 and Japanese Patent Application Publication No. 2004-361514 disclose reflection-type color display media in which a color filter is formed on an electrophoretic element, but it is clear that no good image quality may be obtained when a color filter is formed on a display medium with a low white reflectance and a low contrast ratio.
Furthermore, Japanese Patent Application Publication No. 2004-520621 and Japanese Patent Application Publication No. 2004-536344 disclose electrophoretic elements in which each of particles colored with plural colors is moved to attain coloring, but, even though these methods are used, the above-mentioned problem of a reflectance or contrast ratio may not be solved in principle, and it may not be possible to attain a high white reflectance and a high contrast ratio simultaneously.
Meanwhile, an electrochromic display device is known as a reflection-type display device provided with no color filter. Herein, a device using an electrochromic phenomenon is a promising technique for realizing a reflection-type display device without providing a color filter as mentioned above. An electrochromic display device is expected as a multicolor display device, because it may be possible to develop a wide variety of colors depending on a structure of an electrochromic compound.
A phenomenon such that an oxidation-reduction reaction is caused reversibly by application of an electric voltage whereby a color is changed reversibly is referred to as electrochromism. An electrochromic display device is a display device utilizing color development/color erasing (color development/erasing, below) of an electrochromic compound which causes such an electrochromism phenomenon. Study and development of such an electrochromic display device, such as material development and device designing, have been conducted widely for a potential candidate of a display device technique for an electronic paper application, because it is a reflection-type display device, has an memory effect, and is capable of operating at a low voltage.
However, an electrochromic display device may have a drawback of a slow response speed for color development/erasing due to color development/erasing conducted by utilizing an oxidation reduction reaction in principle. Japanese Patent Application Publication No. 2001-510590 discloses an example contemplated to improve a response speed of color development/erasing by fixing an electrochromic compound near an electrode. As the disclosure of Japanese Patent Application Publication No. 2001-510590 is referred to, a period of time required for color development/erasing, which was about 10 seconds conventionally, is improved so that both a period of time of color development from colorless to a blue color and a period of time of color erasing from a blue color to colorless are about 1 second. However, this may not be sufficient and further improvement of a response speed of color development/erasing is required in study and development of an electrochromic display device.
An electrochromic display device may be capable of developing a wide variety of colors depending on the structure of an electrochromic compound, and hence, expected as a multicolor display device. In particular, a colorless state and a color development state are exchanged reversibly, and hence, a laminated layer multicolor structure may be attained. In a color display with such a laminated layer structure, it is not necessary to divide one picture element into three, i.e., red (R), green (G), and blue (B) as a conventional technique, and accordingly, a reflectance and contrast ratio of a display device are not degraded.
Some examples of a multicolor display device are publicly known which utilizes such an electrochromic display device. For example, Japanese Patent Application Publication No. 2003-121883 discloses a multicolor display device using an electrochromic compound in which fine particles of plural kinds of electrochromic compounds are stacked. Herein, Japanese Patent Application Publication No. 2003-121883 discloses an example of a multicolor display device in which plural kinds of electrochromic compounds are stacked, which are a plurality of polymer compounds having a functional group and exhibiting cooler development at a different voltage, so as to provide multicolor display electrochromic compounds.
Japanese Patent Application Publication No. 2006-106669 discloses a multicolor display element characterized by including a display electrode, an opposing electrode provided to oppose and separate from the display electrode, an electrolyte arranged between both electrodes, and a display layer formed by stacking or mixing two or more kinds of electrochromic compositions developing different colors on a surface of the display electrode at an opposing electrode side, wherein at least either of threshold voltages for providing a color development state, threshold voltages for providing a color erasing state, amounts of electric charge required to develop a color with a sufficient color density, or amounts of electric charge required to erase a color sufficiently are different therefrom.
However, a viologen compound included in an electrochromic composition develops a color such as a blue color or a green color, and it may not be possible to develop a color of cyan required for attainment of full color.
Furthermore, Japanese Patent Application Publication No. 2006-106669 discloses a display device in which a multilayer of electrochromic layers are formed on an electrode and multicolor is color-developed by utilizing a difference between electric voltage values or electric current values required for color development. Herein, Japanese Patent Application Publication No. 2006-106669 discloses an example of a multicolor display device having a display layer formed by stacking or mixing plural kinds of electrochromic compounds developing different colors and having different threshold voltages for color development and amounts of electric charge required for color development.
Moreover, Japanese Patent Application No. 2003-270671 discloses an example of a multicolor display device provided by laminating a plurality of structural units in which an electrochromic layer and an electrolyte are interposed between a pair of transparent electrodes. Furthermore, Japanese Patent Application Publication No. 2004-151265 discloses an example of a multicolor display device corresponding to three colors of RGB in which a passive matrix panel and an active matrix panel are formed by a structural unit described in Japanese Patent Application No. 2003-270671.
However, viologen-based organic electrochromic compounds illustrated in Japanese Patent Application Publication No. 2001-510590, Japanese Patent Application Publication No. 2003-121883, and Japanese Patent Application Publication No. 2006-106669 develop a blue color or a green color and may not develop three primary colors of yellow, magenta, and cyan required for attainment of a full color. In order to the above-mentioned three primary colors, it may be necessary to have a sharp absorption at the time of color development, in particular, it may be necessary to have a sharp absorption in a short wavelength range with respect to yellow color development but no electrochromic compound capable of developing a yellow color and capable of developing or erasing a color stably has been obtained until now.
Furthermore, it may be necessary to have a sharp absorption in a long wavelength range with respect to cyan color development and no electrochromic compound capable of developing a cyan color and capable of developing or erasing a color stably has been obtained until now.
Moreover, no electrochromic compound having absorption all over the wavelength range of visible light and developing a black color has also been obtained until now. If an electrochromic compound developing a black color is provided, it may be possible to manufacture a monochromatic display element with a high contrast.
In addition, a styryl-type coloring agent is used in Japanese Patent Application Publication No. 2003-270671, Japanese Patent Application Publication No. 2004-151265, Japanese Patent Application Publication No. 2008-122578, whereby it may be possible to develop a color in an YMC-system and there is a problem in a color development or erasing stability or repetition durability.
Meanwhile, Japanese Patent Application Publication No. 2007-241238 discloses that a structure with a furan ring, thiophene ring, selenophene ring or alkylpyrrole ring introduced into between two pyridine rings as illustrated therein develops a good magenta color. However, this structure may be yellowish colored at a color erasing state and reduction of its coloring at a color erasing state may be problematic.